1. Field of the Invention
The present invention relates generally to telephony systems.
2. Related Art
Improved voice quality, using wideband speech compression is gaining increasing importance in Internet Protocol (IP) telephony, especially in IP telephones, where this differentiates from traditional analog or digital narrowband phones. Wideband speech compression, where “wideband” represents audio signals sampled at greater than 8 kHz sampling rates, for example, can be characterized as “better than toll quality voice,” or “better voice quality than the Public Switched Telephone Network (PSTN).”
Within a pure IP environment, wideband compressed speech can be carried directly between the two endpoints, for example, between two IP telephones engaged in a voice call. In many situations, however, calls are routed through the traditional PSTN use narrowband data links intended for carrying narrowband telephony. Such routing requires a conversion of the compressed wideband speech to a narrowband format, such as ITU-T Recommendation G.711 μ-law or A-law coded Pulse Code Modulation (PCM) at 64 kilo-bits per second (kbps), which corresponds to speech sampled at 8 kilo-Hertz (kHz) (that is, 8,000 samples per second), and digitized to 8-bits per sample. A data link operating according to such parameters is an example of a narrowband data link. The process of converting to the narrowband format typically includes a process referred to as “transcoding.” The improved quality of wideband speech is lost in this process.
In situations where a caller at one endpoint uses a traditional narrowband telephone, there is little or nothing that can be done because the narrowband telephone frequency band-limits speech signals. However, there are a number of situations (and potentially an increasing number) where callers at both endpoints use IP telephones capable of supporting wideband telephony, and yet the call is still routed through the PSTN. Examples of this include cases where the IP telephony infrastructure is immature (e.g., the billing infrastructure is immature, etc.), and thus the existing PSTN infrastructure (e.g., a Class 5 switch) is used to provide these important services. Other reasons may be that the two endpoints are under the control of different service providers, or managed by different call management domains. Regardless of the reason, the result is the same: The improved voice quality of wideband voice compression is disadvantageously lost due to transmission over narrowband data links.